The left thumb operates both a tone hole and the register key. On some models of clarinet, equivalent to many Albert system clarinets and increasingly some higher end Boehm system clarinets, the sign up key is a 'wraparound' key, with the important thing on the back of the clarinet and the pad on the front. Advocates of the wraparound sign in key say it improves sound, and it is tougher for moisture to accumulate in the tube below the pad. Nevertheless, there is a consensus among repair techs that this sort of register key is more challenging to maintain in adjustment, i. e. , it is tough to have enough spring force to close the outlet securely. The body of a fashionable soprano clarinet is in a position with a large number of tone holes of which seven six front, one back are covered with the fingertips, and anything else are opened or closed using a set of keys. These tone holes let the player produce every note of the chromatic scale. On alto and bigger clarinets, and some soprano clarinets, key coated holes replace some or all finger holes. The most common system of keys was named the Boehm system by its designer Hyacinthe Klosé in honour of flute designer Theobald Boehm, but it is not an analogous as the Boehm system used on flutes. The other main system of keys is named the Oehler system and is used mostly in Germany and Austria see History.

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